I. Field
The invention relates generally to the field of wireless communications, and more particularly to a method, apparatus, and system for efficiently determining ACK to NACK errors in multiple access communication systems.
II. Background
In recent years, communication systems' performance and capabilities have continued to improve rapidly in light of several technological advances and improvements with respect to telecommunication network architecture, signal processing, and protocols. In the area of wireless communications, various multiple access standards and protocols have been developed to increase system capacity and accommodate fast-growing user demand. These various multiple access schemes and standards include Time Division Multiple Access (TDMA), Frequency Division Multiple Access (FDMA), Code Division Multiple Access (CDMA), and Orthogonal Frequency Division Multiple Access (OFDMA), etc. Generally, in a system which employs TDMA technique, each user is allowed to transmit information in his assigned or allocated time slots whereas an FDMA system allows each user to transmit information on a particular frequency that is assigned to that particular user. A CDMA system, in contrast, is a spread spectrum system which allows different users to transmit information at the same frequency and at the same time by assigning a unique code to each user. In an OFDMA system, a high-rate data stream is split or divided into a number of lower rate data streams which are transmitted simultaneously in parallel over a number of subcarriers (also called subcarrier frequencies herein). Each user in an OFDMA system is provided with a subset of the available subcarriers for transmission of information. The subset of carriers provided to each user in an OFDMA system can be fixed or vary, for example, in the case of Frequency-Hopping OFMDA (FH-OFDMA). Multiple access techniques in TDMA, FDMA, and CDMA are illustrated in FIG. 1. As shown in FIG. 1, the communication channels in FDMA are separated by frequencies in which a particular channel corresponds to a particular frequency. In a TDMA system, the communication channels are separated by time in which a particular channel corresponds to a particular time slot. In contrast, communication channels in a CDMA system are separated by codes in which a particular channel corresponds to a particular code.
In wireless systems, it is usually inefficient to guarantee a reliable packet transfer on every single transmission. The inefficiency is particularly pronounced in systems where underlying channel conditions vary drastically from transmission to transmission. For example, in an FH-OFDMA system, there is a wide variation in the received signal-to-noise ratio (SNR) between frames/packets, thus making it difficult and inefficient to guarantee a small frame error rate (FER) for each packet transmission. Such difficulty and in-efficiency also apply to other communication systems which employ orthogonal multiple access techniques including, but are not limited to, TDMA, FDMA, and orthogonal CDMA, etc.
In such communication systems, a packet retransmission mechanism such as the Automatic Retransmission/Repeat Request (ARQ) scheme may be used to help increase efficiencies in message transmissions and to improve packet transmission reliability. A packet transmission acknowledgment is signaled from the receiver to the transmitter using a low-rate feedback channel. Upon successful receipt of such transmissions, the access point typically sends an indicator of acknowledgment (i.e., an ACK message) to the access terminal that the previous transmission is received correctly and that the receiver is ready for a new packet transmission. A negative acknowledgment (NACK), on the other hand, suggests that an error is detected in the previously transmitted packet and that a retransmission is required.
Generally, there are two categories of packet combining techniques: code combing and diversity combining. In code combining systems, sub-packets are concatenated to form noise-corrupted codewords from increasingly longer and lower-rate codes. An example of a code combing technique is the Type-II Hybrid ARQ (H-ARQ) protocol, where the transmitter responds to the retransmission requests by sending additional parity bits to the receiver. The receiver appends these bits to the received packet, allowing for increased error correction capability. In diversity combining systems, the individual symbols from multiple, identical copies of a packet are combined to create a single packet with more reliable constituent symbols
In systems where ARQ is used, the transmitter and the receiver need to remain synchronized in terms of the order in which packets are transmitted. If the feedback channel is error-free, the packet (or sub-packets, in case of retransmission) ordering is implicit. However, such an error-free transmission on the feedback channel cannot be achieved in practice. An error in the acknowledgment can cause packet (or sub-packet) sequencing error at the physical layer.
There are two types of error on the acknowledgment channel: (i) ACK→NACK and (ii) NACK→ACK. A positive acknowledgment mistaken as a negative acknowledgment (ACK→NACK) causes a small loss in throughput, while a negative acknowledgment mistaken as a positive acknowledgment (NACK→ACK) causes a retransmission at a higher layer (e.g., RLP). Both errors, if not detected, can cause the transmitter and the receiver to lose packet-level synchronization.
Often, a sub-packet ID (or at least a one-bit flag indicating an original transmission or a retransmission) is signaled along with each sub-packet to help mitigate this sequencing/synchronization error. Specifically, by looking at the accompanying sub-packet ID, the receiver will be able to detect the out-of-sequence transmission caused by an error on the acknowledgment channel. Unfortunately, such a signaling is rather expensive and is itself unreliable. Often, a disproportionate amount of bandwidth is required in order to transmit the sub-packet ID reliably.
Accordingly, there exists a need to detect an out of sequence transmission caused by the positive acknowledgment being mistaken as a negative acknowledgment without an explicit signaling.